Vice President Joe Biden told Ukrainian political leaders Tuesday that the United States stands with them against "humiliating threats" and encouraged them to root out corruption as they rebuild their government.

In the most high-level visit of a U.S. official since crisis erupted in Ukraine, Biden told leaders from various political parties that he brings a message of support from President Barack Obama as they face a historic opportunity to usher in reforms.

"The opportunity to generate a united Ukraine and getting it right is within your grasp," Biden said. "And we want to be your partner and friend in the project. We're ready to assist."

Biden spoke to nine Ukrainians in a hearing room with gilded moldings at the parliament, or Rada. The group included three candidates running for president in the May 25 election - most notably billionaire chocolate magnate and front-runner Petro Poroshenko. Biden told the candidates he hopes that they have more luck than he did in two presidential runs.

Biden met earlier with acting Ukrainian President Oleksandr Turchynov and planned a later meeting with acting Prime Minister Arseniy Yatsenyuk and democracy activists. After his meetings, Biden plans to announce new technical support to the Ukrainian government for energy and economic reforms.

The vice president's visit comes at a critical time, days after a tenuous international agreement was reached to de-escalate violence in eastern Ukraine, where pro-Russia insurgents oppose the government in Kiev.

Biden told the lawmakers a priority for the U.S. is to help them become independent from Russian energy supplies. "Imagine where you'd stand today if you could tell Russia to keep your own gas," Biden said.

Biden said they have an historic chance now that former President Viktor Yanukovych has fled the country. "This is a second opportunity to make good on the promise of the Orange Revolution," Biden said in a reference to 2004 protests that overturned a widely criticized election that had given Yanukovych the presidency. Yanukovych later took office but left the country after violent protests in February.

Biden added, "You have to fight the cancer of corruption that is endemic in your system right now." He mentioned cleaning up the court system and finding the right balance of power between the president and Rada.

"I want you to know I do not underestimate the incredible pressure you all are under," Biden said. "I do not underestimate the challenges you all face. And I do not underestimate the frustration you all must feel when someone like me comes along to say what a great opportunity this is for you all."

But he added that the upcoming election may be the most important in the country's history. "The truth of the matter is your fellow countrymen expect a whole lot from you right now," he said.

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